This one is new to me.
I'm trying to build Plone 5.1 with dev packages and getting this error.
Specifically I'm working with the buildout that ships with collective.importexport (https://github.com/collective/plone.importexport)
To build against Plone 5.1 I use the command
bin/buildout -c version_plone51.cfg
this is the error I get:
Error: There is a version conflict.
We already have: UNKNOWN 0.0.0
but flake8 3.6.0 requires 'configparser'.